Monochrome icons cause the interface to recede into the background, giving more focus to the content. It's the same logic behind hiding UI elements when not in

Monochrome icons cause the interface to recede into the background, giving more focus to the content. It's the same logic behind hiding UI elements when not in use. This is a good choice in situations where the user is mainly viewing content rather than interacting with the application—for instance, PDF readers and video players. Google probably found that most users use Gmail mainly for reading, rather than processing, emails—and decided that it should more like a viewer and less like an application.